This application relates generally to gas turbine engines and, more particularly, to combustors for gas turbine engine.
Air pollution concerns worldwide have led to stricter emissions standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) generated as a result of gas turbine engine operation. In particular, nitrogen oxide is formed within a gas turbine engine as a result of high combustor flame temperatures. Making modifications to a gas turbine engine in an effort to reduce nitrogen oxide emissions often has an adverse effect on operating performance levels of the associated gas turbine engine.
In gas turbine engines, nitrogen oxide emissions can be reduced by increasing airflow through the gas turbine combustor during operating conditions. Gas turbine engines include preset operating parameters and any such airflow increases are limited by the preset operating parameters including turbine nozzle cooling parameters. For example, increasing airflows within domed combustors including inner and outer swirlers and premixers may cause wake recirculation to develop as airflows exiting the inner swirler separate from the swirler vanes. Furthermore, such wake recirculation permits fuel to dwell within the premixers and potentially autoignite within the premixers. Such autoignition increases emissions from the combustor and may potentially damage components within the combustor. As a result, to increase the airflow within the gas turbine combustor, the gas turbine engine and associated components often must be modified to operate at new operating parameters.
Because implementing gas turbine engine modifications is labor-intensive and time-consuming, users are often limited to derating the operating power capability of the gas turbine engine and prevented from operating the gas turbine engine at full capacity. Such derates do not limit the amount of nitrogen oxide formed as the engine operates at full capacity, but instead limit the operating capacity of the gas turbine engine.
In an exemplary embodiment, a gas turbine engine includes a combustor system to reduce an amount of nitrogen oxide emissions formed by the gas turbine engine. The combustor system includes a combustor including a first annular dome that includes a premixer cup. A centerbody is secured co-axially within the dome and includes at least one orifice for supplying fuel to the dome. An inner swirler is attached to the centerbody and an outer swirler is attached radially outward to the inner swirler such that a leading edge of the inner swirler and a leading edge of the centerbody are disposed a distance upstream from a leading edge of the outer swirler relative to the dome. As a result, a premixing distance measured between the centerbody orifice and an exit of the dome is increased in comparison to known combustor assemblies.
During operation of the gas turbine engine, air and fuel are mixed in the dome prior to the fuel/air mixture exiting the dome for combustion. Although the premixing length is increased because the centerbody is positioned upstream from the outer swirler, because the inner swirler is also positioned upstream from the outer swirler, wake recirculation is reduced and fuel and air thoroughly mix prior to exiting the dome. As a result, nitrogen oxide emissions generated within the combustor are reduced. Furthermore, because wake recirculation is reduced, fuel is prevented from dwelling in the wake recirculation and a potential of fuel autoigniting within the combustor domes is reduced.